The present invention relates to liquid level sensing systems and methods. Particularly, the present invention relates to a system and method of exceptional utility in the sensing of melt level in a silicon crystal growing furnace of the Czochralski type.
The successful growth of silicon crystals using the Czochralski process requires the establishment and control of the correct position for the melt level with respect to the hot zone. Previously, control of this parameter has been established by "dead reckoning" based on measurements of the necessary parameters to relate the mass of the charge loaded into the crucible and the external crucible shaft position, to a known initial melt level. In order to maintain this initial melt level, the cup lift rate for the crucible is calculated as a ratio, (or percent of pull rate) using the crystal diameter, crucible diameter, and density change from solid to liquid. The sensing of the initial melt level is critical in establishing the reproducibility of the crystal growing process in a production mode.
However, the initial position calculation using the "dead reckoning" method can introduce significant errors into the growing process due to the plastic flow of the quartz crucible to conform to the graphite cup at the high temperatures encountered. Further, there always exists the possibility of the operator's mental errors in calculation.
The control of melt level during the growing process suffers from all of the aforementioned uncertainties as well as requiring updated corrections for variations in crystal diameter. Additionally, the fact that a small error in the calculated ratio produces a cumulative error in the long growth cycles typical of today's large furnaces, further renders the "dead reckoning" method unacceptable. Typically, failure to accurately control the melt level will result in crystal yield losses due to either loss of crystal structure or poor diameter control since, in the latter instance, the diameter sensor is dependent on maintaining a fixed melt level, (or at least a known melt level) which level can be used to correct the diameter sensing system. As a practical matter, a crystal diameter less than that desired results in wholly wasted crystal material, while a crystal diameter exceeding that desired will require grinding to bring it down to the correct size. In this latter instance, there is still a wastage of material as well as the time and effort involved in grinding.
Overall, the need for improved melt level control has become more and more important and with larger, long cycle furnaces, it has become an absolute necessity for automating the crystal growing process. In this regard, U.S. Pat. No. 3,740,563 which issued to Reichard, describes an electro-optical system and method for sensing and controlling the diameter and melt level of pulled crystals. This patent describes a system which reflects a narrow beam of light off the meniscus occurring at the periphery of the growing crystal with the liquid melt surface. The reflected beam therefrom is detected with a two-axis spot locator. According to the system described, movement of the reflected beam in a tangential direction is primarily due to changes in melt level, while movement in a radial direction is primarily due to changes in diameter. While using the small meniscus area provides a relatively vibration free reflective surface, the system described cannot however, be used to establish initial melt level due to vibrations or waves on the free melt surface. Further it would, in fact, have problems when used in conjunction with out of round or faceted crystals or crystals which may orbit in a slightly eccentric manner as is common in the Czochralski process employing cable or chain lift pulling mechanisms. Further, the use of the chopped light source and synchronized detector circuit described does not result in as good a signal to noise ratio as is achieved by using the extremely high intensity, narrow pass optical system of this invention.
U.S. Pat. No. 3,574,650 issued to House, describes a vacuum deposition system for the control of the location of the evaporation source. This patent describes the use of a laser to reflect a beam of light off of a small metallic melt used as a vapor source in a vacuum deposition process. While the use of a small diameter monochromatic light source such as a laser coupled with a filter would improve the signal to noise ratio of the system, the system is restricted to small surface tension stabilized melts due to the small beam diameter and its vulnerability to being seriously disturbed by melt vibrations. The system described is therefore concerned with measuring an angle of incidence and reflection on a small stabilized melt surface and is not an averaging system which can deal with the ripply surface of a large unstable melt surface.
Thus, it is apparent that there remain several problems in the accurate sensing of melt level which have not been addressed by the aforementioned patents or other systems hereinbefore introduced. Firstly, the light beam reflecting off of the melt surface is accompanied by the intense thermal radiation emitted and reflected from the melt surface thus creating a significant signal to noise ratio problem in detecting a reflected beam from the melt surface. Secondly, the surface of the melt does not resemble a flat mirror, but rather is disturbed by ripples and waves due to turbulent convection and other mechanical disturbances resulting from cup rotation and erratic wetting of the crystal at the crucible. Normally, a beam deviation of up to several inches at the view port can be expected for a small beam under these conditions. Finally, any optical measurement of the melt level must contend with changes in the transmission of the furnace view ports due to the condensation of oxides and dopants thereon as well as possible light source variations.
It is therefore an object of the present invention to provide an improved melt level sensing system and method.
It is further an object of the invention to provide an improved melt level sensing system and method which will accurately determine the initial melt level in a Czochralski crystal growing process.
It is still further an object of the invention to provide an improved melt level sensing system and method which provides a good signal to noise ratio in the presence of intense thermal radiation.
It is still further an object of the invention to provide an improved melt level sensing system and method which will accurately determine melt level in the presence of surface waves and ripples due to turbulent convection and mechanical vibrations due to cup rotation or other causes.
It is still further an object of the invention to provide an improved melt level sensing system and method which allows for the accurate sensing of melt level despite the partial obscuration of the furnace view ports due to the condensation of oxides and dopants or the presence of light source variations.